The present invention relates to a process which allows to obtain very pure and with high yields CHF2xe2x80x94CF3 (HFC 125) pentafluoroethane.
HFC 125 is an harmless fluorocarbon for the ozone layer, therefore meeting the requirements of the Montreal Treaty. For the commercial uses of the compound an high purity is required.
The possibility to obtain pure pentafluoroethane depends on the type of impurities which are formed during the synthesis. For example CFC 115 (chloropentafluoroethane CF2Clxe2x80x94CF3) is an impurity which can be eliminated with difficult from HFC 125, therefore its presence does not allow to obtain the compound at a very pure level. In order to produce pentafluoroethane meeting these requirements, processes must be employed wherein CFC 115 does not form or is formed only in traces.
The industrial processes to produce HFC 125 usually utilize HCFC 124 (tetrafluorochloroethane C2HF4Cl) as starting compound. HCFC 124 is subjected to fluorination with HF, on suitable catalyst, or is transformed (by dismutation) into a mixture of HFC 125+HCFC 123 (dichlorotrifluoroethane C2HF3Cl2) by operating at a suitable temperature and in the presence of a catalyst. In the patent application WO 95/16654 a process for obtaining pentafluoroethane is described starting from a previously obtained gas mixture and containing as main component chlorotetrafluoroethane and lower amounts of chlorofluorocarbons (CFCs) with two carbon atoms. In a first step CFCs are separated, so as to have HCFC 124 substantially pure for the reaction with HF, in particular free from dichlorotetrafluoroethane (C2Cl2F4) CFC 114, which under these conditions would react forming CFC 115. Therefore this patent discloses that in order to obtain pure HFC 125 by reacting HF with HCFC 124, the starting compound must be previously purified by removing the impurities of chloroflurocarbon C2 (114).
The dismutation process of HCFC 124 is more suitable with respect to the reaction with HF since the starting compound purity is less criticl, further the selectivity is higher. This process has the drawback that the HCFC 124 conversion into HFC 125 is limited by the increase of the reaction by-products amounts. See EP 569,832 in the name of the Applicant.
The need was felt of a process for producing HFC 125 starting from HCFC 124, wherein it was possible to increase the amount of the produced HFC 125, reducing the amount of impurities in comparison with the fluorocarbon synthesis described with the known processes.
It has been surprisingly and unexpectedly found by the Applicant that it is possible to solve the above problem, and this is an object of the invention, using a gaseous process, wherein pentafluoroethane is obtained by dismutation of chlorotetrafluoroethane, in the presence of a catalyst comprising a mixture of trivalent chromium oxide with at least an alkaline-earth metal oxide selected from Mg, Ca, Sr and Ba.
The reaction temperature is in the range 150xc2x0 C.-250xc2x0 C., preferably 180xc2x0 C.-240xc2x0 C.
The contact time with the catalyst, determined as the ratio between the catalyst volume and that of the gas flow at the working temperature and pressure is in the range 5-30 seconds, preferably 10-20 seconds.
The pressure is not critical, but preferably is comprised between 1 and 10 bar.
The reaction is carried out by flowing gaseous HCFC 124, optionally diluted with an inert gas such as for example nitrogen, through the catalyst.
Preferably the reaction is carried out in a fluidized bed; in this case the catalyst particles must have sizes suitable for this kind of plant.
The g atoms ratio between the chromium and the alkaline-earth metals ranges from 50:1 to 3:1, preferably from 20:1 to 5:1.
The catalyst is preferably supported.
Preferably the catalyst support is aluminum fluoride obtainable by alumina fluorination, and having a fluorine content not lower than 90%, preferably not lower than 95%, with respect to the stoichiometric.
Generally the used AlF3 is mainly formed of gamma phase, as described in FR 1,383,927, and has a surface area generally in the range 25-35 m2/g. If the catalyst is used in a fluidized bed the support has the granulometry suitable for this kind of reactor, as well known to the skilled in the field.
In the supported catalyst the sum of the percentages of the contained chromium and alkaline-earth metal is in the range 5-15% by weight, preferably 10-15%.
The catalyst is preferably prepared by impregnation of the support with an aqueous solution of a soluble chromium and of the alkaline-earth metals salt. The support impregnation can be carried out with any method known in the prior art, for example with the method known as dry impregnation, for example as described in EP 408,005, herein incorporated by reference.
According to this method the impregnation is carried out by pouring on the support, in sequence, according to the process described hereinunder, portions of an impregnating solution, such that the global volume is not higher than the volume of the aluminum fluoride pores. The solution for the impregnation is prepared by dissolving in water the required amounts of the corresponding salts, preferably chlorides, of the trivalent chromium and of the alkaline-earth metals. The solution is poured in portions on the support, drying at 110xc2x0 C. for some hours after every addition, in order to evaporate the water from the support pores.
At the end of the impregnation, the catalyst must be activated: the operation can be directly carried out in the reactor used for the dismutation, by calcining in inert gas current, at the temperature of about 400xc2x0 C. for 4-8 hours and then treating at 360xc2x0 C. with anhydrous HF for 12-24 hours.
Some Examples are given for illustrative purposes and they are not limitative of the employment possibility of the invention.